Spring Rates

[RossD 43]

Any suspension gurus out there?!?!

I've been looking into springs recently for the 205. I'm a little confused about the spring rates given for some aftermarket springs. (That is where they are given!)

 

In the french RTAI manual for the 205, it gives the different spring rates for the 205 petrol models, all fine you might say, but it gives them in two sections. One is the spring rate of the spring and the other is the spring rate at the wheel.

 

When looking at aftermarket springs, are they giving the spring rate at the wheel or of the spring? For example, Sandy reckons the Eibach 7001 dual rate 106 springs are about 115/155 lb/in. I'm only mentioning, as the book says that the spring rate at the wheel for the GTi models works out about 160 lb/in, so in my eyes using this spring the car would be "softer" than OE.

Has anyone used the Eibach springs and found them to be stiffer than the original GTi springs?

 

I should mention I'm looking at the standard 6" type of springs and not the 2 - 3" coilover types.

 

Cheers

 

Ross

Edited November 5, 2007 by RossD

[SPGTi 2]

Taken from the link in a recent thread I started (Useful information on suspension setup)

 

Wheel Rate (Spring Value at Wheel)

 

This is the true value of the spring at the wheel (where it counts), if we had a motion ratio of approx 0.167, that would meant fitting a 600lb per inch spring to produce 100lb per inch at the wheel. The wheel rate can be worked out from the motion ratio and the used spring rate, e.g. 600lb per inch spring multiplied by 0.167 the motion ration = 100.2lb per inch at the wheel.

 

Motion ratio

 

The difference in movement between the suspension and the wheel, or the leverage applied to the suspension by the wheel movement. A good example of a one to one motion ratio is a “Po-go-stick”, as you move up and down the spring is moving the same amount. A “McPherson” type strut generally has a one to one ratio, or very close to one to one, whereas a coil over damper on a “double wish bone” set-up may vary a huge amount. The front of a Lotus Seven or similar with double wish bones will generally have a coil over damper with the top mounting point close to the upper chassis rail, so when viewed from the front the dampers lean in towards the engine. This increases the “motion ratio”, let’s say for ease that the wheel moves two inches to one inch at the spring, so as the spring compresses one inch the wheel move two inches (or if we look at it the other way round, the spring moves half of the wheel. The ratio is 0.5). If we were to fit a 200lb per inch spring the wheel rate would be 100lb per inch. So to calculate the motion ratio you need to measure pivot point of suspension arm to centre of spring position along the arm then measure pivot point to ball joint (in other words the length of the suspension arm), divide the pivot to spring length by the overall length squared (or multiply the answer by itself. For example 12” from pivot to centre of spring with an arm length of 20” would be 12 divided by 20 = 0.6 squared (0.6 x 0.6) = Motion ratio of 0.36 (If the wheel moves 1” the spring moves 0.36”). So if we want a wheel rate of 100lb per inch divide 100 by 0.36 = 277.77, so I would need a 278lb per inch spring to get a 100lb per inch at the wheel. I will go into more details on spring rates later on, for now we need to get the bigger picture.

 

 

Steve

[Sandy 189]

I've had a recount on that spring rate btw, I now think it's more like 70-200lb/inch. Either way, it seems to work very well.

[RossD 43]

I've had a recount on that spring rate btw, I now think it's more like 70-200lb/inch. Either way, it seems to work very well.

 

Ahhhh perfect! I thought it was odd that a lowering spring for a 205 was actually softer than the original!

I'll be going for these now then